The present invention relates to a Micro-Electromechanical System (MEMS) metrology device and techniques for using the device to measure geometric, dynamic, and material properties of MEMS devices.
MEMS devices are known. MEMS devices typically include a two-and-one-half dimension (2.5D) structure that has movable and anchored portions. A common MEMS device having such a structure is a MEMS accelerometer such as the NASA Electronic Part and Package (NEEP) 2001. Such a device includes a generally planar structure having one or more beams, one or more springs, stationary polysilicon fingers and capacitive sense plates. Such devices are made from poly or single crystal silicon as well as from SiGe and SiC and other silicon-based materials. It has been determined that the performance of such devices changes in time due in part to changes in their material properties, which result in part from changes in thermal and load or shock cycles. There is therefore a need for monitoring the changes in material and geometric properties of MEMS devices.
In the CMOS world E-test devices are available that allow for various in-situ measurements. For example, E-test resistivity bridges can be used to measure resistivities. Also, E-test devices are available for the in-situ measurement of device line widths. So, while in the CMOS world E-test devices are available that can be incorporated in the masks used to manufacture the CMOS devices, no such equivalent in-situ device exists in the MEMS world. This lack of in-situ measurement capability for MEMS devices is further complicated due to the fact that MEMS devices are movable and that an aspect of their related measurement is directed to measuring the mechanical properties of the MEMS devices.
Various approaches are currently available for the measurement of parameters related to MEMS devices. Some of these approaches involve the use of Scanning Electron Microscopy (SEM), optical microscopy, interferometry, surface profileometry and nanoindentation. These approaches tend to be very expensive to implement requiring the use of expensive equipment and highly qualified operators. Another known test is the M-test, which involves using a test chip having arrays of cantilevered beams that are electrostatically actuated. Such a test is sensitive to process variations and cannot measure geometric properties, such as the line width of the device. Another test uses the out-of-plane bending of a cantilevered beam. This test is however restricted to the measurement of out-of-plane characteristics of a beam.
There is therefore a need for a tool for monitoring the material and geometric properties of MEMS devices that does not suffer from the above shortcomings.